1. Field of the Invention
This invention relates to lasers. More specifically, this invention relates to coupled waveguide laser arrays.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Gas lasers, particularly CO.sub.2 gas lasers, are utilized in a variety of industrial, medical and military applications. Nonetheless, gas lasers are known to have some limitations. For example, conventional gas lasers include a sealed glass tube, a cooling jacket, cavity mirrors, and discharge electrodes with associated driving circuitry. The electrodes are positioned within the glass tube and excite a gas mixture through a longitudinal DC discharge. This common excitation method tends to affect the composition of the gas mixture adversely. That is, depletion of oxygen from the typically CO.sub.2 based gas mixture tends to reduce the operational lifetime of the laser.
Further, conventional gas lasers are inappropriate for certain applications as a result of their fragility and large size and weight.
Waveguide lasers were initially developed in an effort to provide more durable and compact sources of laser light. Conventional waveguide lasers include a hollow bore waveguide enclosing an active (gas) medium, reflecting mirrors, and a typically multiple electrode configuration. However, the direct current discharge typically required to energize conventional waveguide lasers has also been found to lead to oxidation of the active medium as discussed above.
Transverse radio frequency (rf) excited waveguide lasers provide a number of advantages relative to conventional waveguide lasers. Structurally, rf-excited waveguide lasers typically consist of two ceramic plates sandwiched between a pair of electrodes thereby forming a hollow bore waveguide. An active medium is enclosed within the bore, and resonator mirrors are generally disposed at two ends of the waveguide. Auxiliary shunt inductors appended to the waveguide structure complement the capacitance provided by the parallel electrodes to form what may be envisioned as a resonant electrical circuit. The frequency of the rf excitation signal is chosen to be the resonant frequency of the "circuit".
This method of excitation does not require insertion of an electrode into the active medium and hence obviates the contamination difficulties discussed above. Further, the magnitude of the rf voltage applied to the electrodes is generally significantly lower than the magnitude of voltage in the discharges mentioned above. Consequently, the excitation circuitry of rf-excited waveguide lasers is typically less complicated than that of d.c. actuated waveguide lasers.
Though offering a variety of improvements over waveguide lasers stimulated by a d.c. discharge, single bore RF excited waveguide lasers are typically limited to low power (less than 30 watts) applications. It is known that the optical output power of single bore waveguide lasers may be increased to some extent through increases in bore length. However, the resonating mirrors at either end of the bore may be damaged as a result of excessive intra-cavity cavity laser power density.
Efforts to increase the intensity of beams generated by rf excited waveguide lasers have included the implementation of "folded resonator" mirror geometries. In these "folded waveguide" lasers a plurality of mirrors are used to induce optical energy within the waveguide bore to oscillate in a number geometric patterns. A primary advantage of folded waveguide lasers is that optical power output may be increased without increasing the overall length of the laser. However, the optical power handling capability of the resonator mirrors limits the maximum available output power of folded waveguide lasers to that of single bore waveguide lasers.
Waveguide laser arrays have been developed as an alternative for the attainment of relatively high output power from a compact laser source. Waveguide laser arrays typically include a dielectric layer sandwiched between a pair of electrodes (see e.g., Cantor et al.; "Coupled High Power Waveguide Laser Research"; Interim Report, United Technologies Research Center East Hartford, Connecticut; October 31, 1986.) The dielectric layer includes multiple hollow bores. Portions of internal surfaces of each bore provide electrodes. Resonating mirrors are generally positioned adjacent to apertures at either end of each bore and are oriented perpendicular to the longitudinal axis of the bores.
In this configuration, increases in the number of elements (proportional to output power) in the waveguide laser array result in proportional increases in the effective area of the resonator mirrors. Hence, an advantage of waveguide laser arrays is that output power may be increased without increasing the intensity (power/area) of optical power incident on the resonator mirrors.
In order to obtain optimum far field beam patterns from waveguide laser arrays it is necessary for the individual array elements to oscillate with the same frequency and phase. As a consequence, a number of schemes have been proposed for phase locking waveguide laser arrays. For example, phase locked operation has been obtained for a pair of adjacent CO.sub.2 waveguide lasers separated by a plate of ZnSe which allows a portion of the radiation from one of the waveguides to leak into the adjacent waveguide (D.G. Youmans; "Phase Locking of Adjacent Channel Leaky Waveguide CO.sub.2 Lasers"; Applied Physics Letters. Vol. 44 (4), February 15, 1984). However, a disadvantage of this scheme is that the ZnSe window is exposed to ion bombardment and heating by the laser discharge. Alternatively, as disclosed by L.A. Newman, et al. in "High Power Coupled CO.sub.2 Waveguide Laser Array", Applied Physics Letters, Vol. 48 (25), June 23 1986, phase locked operation has been demonstrated in a guide structure termed a "hollow-bore ridge waveguide". In this structure coupling of radiation between waveguides occurs through simple longitudinal slots running the length of the guide.
Coupled waveguide laser arrays such as those described above may oscillate in a variety of phase locked conditions. In many applications a particularly desirable oscillation condition is one in which the outputs from each bore are of a common optical phase. This manner of oscillation is often referred to as the "lowest order supermode" of the array and results in output of a single, narrow beam. Alternatively, the array is characterized as oscillating in the "highest order supermode" when adjacent waveguides phase lock 180 degrees apart. The output of the array in this latter case includes a pair of beams oriented in slightly different directions, a less useful beam pattern for most applications. However, in certain waveguide laser arrays (e.g. in the array of Newman et al.) the highest order supermode is more easily excited than the lowest order supermode, notwithstanding the comparatively greater utility of the beams produced by arrays oscillating in the latter mode.
Hence a need in the art exists for an improved waveguide laser array which provides a set of in-phase, phase locked optical beams.